The present invention relates to a removing device for removing sludge or scale, for example, on a tube plate and a tube nest of a heat exchanger such as a steam generator used in a nuclear power plant or the like.
In order to understand the present invention, a description of a steam generator used in a nuclear power plant or the like is necessary. Thus, such a steam generator will be briefly described with reference to FIG. 10.
In FIG. 10, numeral 1 indicates a steam generating portion of a steam generator. The steam generating portion 1 has a substantially cylindrical body portion 2 in which hand holes or inspection holes (only one of which is indicated at numeral 17) are diametrically opposed to each other. In the lower portion of the body portion 2, there is arranged a tube plate 3, which defines, together with a bottom portion 2a of the body portion 2, a water chamber 4. Above this tube plate 3, a large number of normally U-shaped heat-transfer tubes 5 are arranged so as to communicate with the water chamber 4. Further, a plurality of tube support plates 6a, 6b, . . . , 6f are horizontally arranged so as to traverse the large number of heat-transfer tubes 5 and laterally support them. Each of the heat-transfer tubes 5 extends vertically through oddly shaped holes usually called BEC (Broached Egg Crate) holes formed in the tube support plates 6a, 6b . . . , 6f. In FIG. 10, some of the BEC holes formed in the tube support plate 6a are schematically shown and indicated by numeral 7.
The space in the water chamber 4 is divided into a hot leg portion 4a and a cold leg portion 4b by a partition 8. One end of each U-shaped heat-transfer tube 5 communicates with the hot leg portion 4a, and the other end thereof communicates with the cold leg portion 4b. Thus, the tube plate 3 also has a large number of holes which are schematically indicated by numeral 3a and which serve to receive the end portions of each of the heat-transfer tubes.
Further, in FIG. 10, numeral 9 indicates a man-hole which enables an operator to enter the water chamber 4, numeral 10 indicates a coolant inlet nozzle communicating with the hot leg portion 4a, numeral 11 indicates a coolant outlet nozzle communicating with the cold leg portion 4b, and numeral 12 indicates trunnions for suspending the steam generator. Though not shown, a steam outlet is formed at the top of the steam generator. Further, above the steam generating portion 1, there is provided a water supply nozzle (not shown) for introducing supply water into the body portion.
In the case, for example, of a steam generator of a nuclear power plant, coolant at high temperature and high pressure supplied from the nuclear reactor enters the hot leg portion 4a of the water chamber 4 through the coolant inlet nozzle 10 and flows through the heat-transfer tubes 5 to reach the cold leg portion 4b before it is returned to the nuclear reactor by way of the coolant outlet nozzle 11. Supply water from the water supply nozzle is supplied into the body portion 2 to fill the periphery of the heat-transfer tubes 5. This supply water is heated by the high-temperature/high-pressure coolant flowing through the heat-transfer tubes 5 to become steam, which goes out through the steam outlet and is supplied to a steam turbine (not shown) for power generation.
As a result of the conversion of supply water into steam, impurities called scale are likely to adhere to the top surface of the tube plate 3, in particular, to the portions of the top surface of the tube plate 3 in the vicinity of the holes 3a into which the end portions of the heat-transfer tubes 5 are inserted and to the peripheral surfaces of the lower portions of the heat-transfer tubes 5 embedded in the tube plate 3. In this specification, these regions will be referred to as xe2x80x9cthe portion centered about the tube platexe2x80x9d. Further, scale is likely to adhere to the upper and lower surfaces of each tube support plate, the regions near the portions directly below the BEC holes 7. In this specification, these regions will be referred to as xe2x80x9cthe portions centered about the tube support platesxe2x80x9d. If neglected, this scale will lead to corrosion of the heat-transfer tubes, etc. Thus, it must be periodically removed.
In this way, steam generators, which find relatively wide use in various fields of industry, involve, as they are used, the generation of impurities called scale on the heat-transfer tubes 5, the tube plate 3, the tube support plates 6a, 6b, . . . , 6f, etc. To maintain the performance of the steam generator and to prevent corrosion or the like due to scale, it is necessary to remove this scale.
For this purpose, various scale removing devices have been developed and proposed. In the case of a steam generator for a nuclear power plant, care must be taken, from a health care perspective, that an operator is not exposed to a dose of radiation greater than a predetermined level. Further, despite the fact that a steam generator is an apparatus of a considerably large size, the very large number of heat-transfer tubes are packed very tight. Furthermore, each heat-transfer tube has a diameter as small as approximately 20 mm, and is relatively thin-walled. Thus, the heat-transfer tubes are subject to damage if a heavy impact is applied thereto. In addition, a plurality of tube support plates are provided in the steam generator, and the distance between adjacent tube support plates is short in comparison to the height of the entire steam generator. These conditions have to be taken into account when developing a scale removing device.
As described above, a steam generator has only one tube plate, whereas it has a plurality of tube support plates. Thus, to clean the areas centered about the tube support plates, the cleaning head of a scale removing device has to ascend and descend through flow slots formed in the tube support plates (e.g., thin and narrow slots 13 formed in the tube support plate 6a shown in FIG. 10). In view of this, a scale removing device for cleaning the portion centered about the tube plate and a scale removing device for cleaning the portions centered about the tube support plates have been separately developed.
FIG. 11 shows a typical scale removing device for cleaning the portion centered about the tube plate, as disclosed in Japanese Patent Application Laid-Open No. 4-503564. The scale removing device, generally indicated at 15, is laterally inserted into a tube lane 16 directly above the tube plate 3 of a steam generator through a hand hole or inspection hole 17. The device includes a transporter 20 adapted to move along a support rail 18. A flexible lance 19 extends through this transporter 20 to a desired cleaning position between tube rows.
FIG. 12 shows in detail the flexible lance 19 and, in particular, a cleaning head 26 provided at the forward end thereof. The flexible lance 19 includes four high-pressure hoses 22 fixed together by a member 25 called a hose bar structure, a nitrogen purge line 23, and a video probe optical fiber cable 24, the forward ends of these components being connected to the cleaning head 26. Provided at the forward end of the cleaning head 26 are a plurality of nozzles 27 communicating with the high-pressure hoses 22. During cleaning, water 28 is jetted out from these nozzles (See FIG. 11).
FIG. 14 shows a typical example of a scale removing device for cleaning the portions centered about the tube support plates, as disclosed in Japanese Patent Application Laid-Open No. 9-026107 by the present applicant. This scale removing device, generally indicated at 30 in FIG. 14, is provided with an insertion mechanism 31 and a guide mechanism 32. The insertion mechanism 31 has on the operator side a driving device 33. Further, it has a fixation plate 34 at its forward end, which is fixed in position by the operation of a second jack 35. A receiving plate 36 and a first jack 37 are provided between the operator-side end and the forward end, whereby the direction of the guide mechanism 32 is changed upwards by 90 degrees. The guide mechanism 32 is divided into a large number of strip-like holding plates 38 so that the mechanism can move in a curve, and a high-pressure hose and an electric wire cable 29 can be mounted to the backsurface of the guide mechanism 32. A cleaning portion main body 40 is provided at the forward end of the guide mechanism 32, and a cleaning head 41 provided there is capable of rotating by 180 degrees.
Though not shown, this cleaning head 41 also has a plurality of nozzles, from which water is ejected to a portion to be cleaned. Through expansion and contraction of this guide mechanism 32, the scale removing device is passed through the flow slots 13 of the tube support plates to remove the scales in the areas centered about the plurality of tube support plates.
Regarding the removal of scale, there has been developed, apart from the technique which removes adhering scale by a water jet from the lance, a technique which aims to prevent adhesion of scale. Thus, in recent steam generators, the scale adhesion range is smaller as compared with that in conventional steam generators. Further, the adhesion thickness is much smaller. On the other hand, the adhesion force of adhering scale is very strong, and it is difficult to remove adhering scale by the above-mentioned water jet. FIG. 15 is a diagram showing an example of how scale adheres to a heat-transfer tube on the tube plate. Shown on the left-hand side of the center line of the heat-transfer tube is the scale (hard scale) adhering condition in a conventional steam generator, and, shown on the right-hand side thereof is the scale (hard scale) adhering condition in a recent steam generator.
In the prior-art techniques, to remove strongly adhering scale (called hard scale), the pressure of the water jet is increased. This has proved effective to some extent for scale which is in the water jet path. However, this is not effective at all for scale which is out of the water jet path, and such scale is allowed to remain. To remove such remaining scale, an attempt has been made to enlarge the width of the water jet path by providing a plurality of washing nozzles or to change the direction of water jets from the nozzles. However, the increase in ejection area has only resulted in a reduction in impact per unit area. Thus, removal of the remaining scale could not be realized as desired.
This will be explained in more detail with reference to FIG. 16. FIG. 16(a) is a perspective view showing an example of the cleaning head used. The cleaning head has eight nozzles A1 through A4 and A5 through A8 in the upper and lower portions of the front surface A thereof, and four nozzles B1 through B4 (nozzles B3 and B4 are not shown) at upper and lower positions in the side surfaces B thereof As shown in FIG. 16(b), this cleaning head is inserted between heat-transfer tubes 5, and water jets 28A and 28B are ejected from each of the nozzles of the front surface A and the side surfaces B of the cleaning head while drawing the cleaning head in the direction of the arrow to thereby perform cleaning. In this case, the regions where the water jets 28A and 28B hit are the regions that can be cleaned, and the other regions are out of the ejection paths. In FIG. 16(b), symbol Y indicates a direction which is perpendicular in a plane to the X-direction in which the tube lane 16 extends. Thus, numerals Y14, Y15, and Y16 indicate the fourteenth, fifteenth, and sixteenth heat-transfer tubes of the tube lane 16.
FIG. 16(c) is a plan view showing the ejection paths of the water jets 28A and 28B ejected as shown in FIG. 16(b). The central, wide shaded area is the region that can be cleaned by the water jets 28A, and the relatively narrow shaded areas on the right and left-hand sides thereof are regions that can be cleaned by the water jets 28B. It can be seen, in plan view, that there exist on the tube plate 3 regions which are out of the ejection paths. It has been difficult to remove scale in these regions out of the ejection paths. In FIG. 16(d), the region that can be cleaned by the water jets 28B are shown in shaded areas with respect to the height direction of the heat-transfer tube 5. It can be seen from this drawing that there also exist regions out of the ejection paths with respect to the height direction of the heat-transfer tube 5. In the prior art, it has been difficult to remove scale in these regions out of the ejection paths. In FIG. 16(d), the numerals given to the extension lines from the shaded areas indicate the numbers of the nozzles in the side surfaces B.
Accordingly, it is an object of the present invention to provide a scale removing device which makes it possible to solve the following problems in existing scale removing devices or which can satisfy the following requirements for existing scale removing devices:
(1) In order to remove the remaining hard scale, it is necessary for the cleaning liquid ejection range to cover the entire tube plate surface and a portion of the heat-transfer tube side wall up to a height of 10 mm or more as measured from the tube plate (including the regions which are out of the ejection path in the existing scale removing devices).
(2) The influence of ejection impact on the material is evaluated by using a locally strong portion as a reference, so that, for the cleaning effect to be high over a wide range, there should be no portion within the ejection range where the cleaning power is locally high. Otherwise, the cleaning conditions would be rather lenient.
(3) The effective cleaning diameter of an in-air water jet is small, so that it is difficult to enlarge the cleaning range with a cleaning nozzle incorporated in the lance.
To achieve the above object, there is provided, in a first aspect of the present invention, a scale removing device for a steam generator of the type which includes a body portion, a tube plate and a plurality of tube support plates arranged horizontally in the body portion so as to traverse the body portion. A plurality of heat-transfer tubes extend from the tube plate and end at the tube plate and extend through the tube support plates in a row-like fashion so as to define a tube lane in the body portion. A hand hole is formed at a position above the tube plate, through which the scale removing device is inserted to clean a portion centered about the tube plate. The scale removing device comprises a flexible lance holding a high-pressure water hose movable with respect to the tube plate and the heat-transfer tubes and a cleaning head mounted to the forward end of the flexible lance. Formed inside the cleaning head are a fluid passage communicating with the high-pressure water hose, a chamber communicating with the fluid passage, and a cavitation generating nozzle hole communicating with the chamber.
The scale removing device preferably further comprises a suspension guide device horizontally arranged above the tube plate and a lance conveying member suspended by the suspension guide device and guided in horizontal movement through the hand hole. The flexible lance runs through the lance conveying member and is paid out from and drawn back to the lance conveying member. The forward end portion of the cleaning head is formed in a curved surface not interfering with the body portion wall surface defining the hand hole.
Further, it is desirable that the lance conveying member have a cylindrical main body through which the flexible lance runs and that the forward end portion of the cleaning head is configured such that when the flexible lance has been completely drawn back, it does not protrude from the peripheral surface of the cylindrical main body. As desired, it is possible to form one or a plurality of the cavitation generating nozzle holes.
To achieve the above object, there is provided, in a second aspect of the present invention, a scale removing device for a steam generator including a body portion, a plurality of tube support plates horizontally arranged inside the body portion so as to traverse the body portion, and a plurality of heat-transfer tubes extending in a row-like fashion through the tube support plates so as to define a tube lane in the body portion. A pair of hand holes are formed in the body portion at a position directly above at least one of the tube support plates so as to be diametrically opposed to each other. Flow slots are formed in the portions of the tube support plates corresponding to the tube lane, the scale removing device being inserted through the hand holes, ascending and descending through the flow slots formed in the tube support plates, and cleaning the portions centered about the tube support plates. The scale removing device comprises a flexible lance holding a high-pressure water hose movable with respect to the tube support plates and the heat-transfer tubes, and a cleaning head mounted to the forward end of the flexible lance. Formed inside the cleaning head are a fluid passage communicating with the high-pressure water hose, a chamber communicating with the fluid passage, and a cavitation generating nozzle hole communicating with the chamber.
Further, the cleaning head includes a head main body portion in which the fluid passage and the chamber communicating with the fluid passage are formed, and a nozzle tip detachably mounted to the head main body portion, the cavitation nozzle hole being formed in the nozzle tip. It is possible to mount an ultrasonic oscillator to the nozzle tip.
Further, the nozzle tip includes a hexahedron defining the cavitation generation hole, thin-plate-like mounting portions extending from two opposing surfaces of the hexahedron except for the surfaces where there are a small-diameter end and a large-diameter end of the cavitation generation hole, and a protruding portion defining a flow passage communicating with the small-diameter end of the cavitation generation hole. A recess of a contour corresponding to the configuration of the nozzle tip is formed in the head main body portion. The nozzle tip can be inserted into the recess and mounted to the head main body portion by the mounting portion. Further, a CCD camera can be suitably provided in the forward end portion of the cleaning head.